New Central Heating Boiler - Help

[bunglesprout]

I moved house a couple of years ago, the house has an old conventional boiler system with separate hot water storage and tank in loft. It is inefficient and will be on it's last legs and need replacing soon. My question to those in the know, is what sort of boiler to replace it with?
The house is a 1920's 4 bed detached, all bedrooms en suite with 3 electric showers and one mixer. Myself, wife and two daughters live there, so potential demand for hot water all at the same time is low. A quick mental tot up I think there are 19 radiators in the house, some have been replaced with modern equivalents, perhaps half are still the old 70's style which I assume were fitted with the boiler.

I know a combi is out and I wouldn't have one anyway. Do I replace with a conventional boiler or system boiler? I am unclear of the advantages or disadvantages of a system boiler over a conventional boiler and the likely costs.